An electromagnetic actuator for actuating a cylinder valve in a piston-type internal combustion engine essentially consists of two spaced-apart electromagnets with opposite-arranged pole faces, between which an armature acting upon the cylinder valve to be actuated can move back and forth, counter to the force of at least one restoring spring, between an open position and a closed position for the cylinder valve. One of the electromagnets thus functions as closing magnet for holding the cylinder valve in the closed position, counter to the force of an opening spring, while the other electromagnet functions as opening magnet for holding the cylinder valve in the opened position by means of the armature and counter to the force of the associated closing spring.
The arrangement in this case is such that the idle position of the armature is the position in the center between the two pole faces. If current is supplied alternately to the two electromagnets, the armature comes to rest against the pole face of the respective electromagnet under current, thus the catching electromagnet, counter to the force of a restoring spring. If the holding current at the respective holding electromagnet is switched off, the armature is accelerated by the force of the restoring spring in the direction of the respective other electromagnet, which is admitted with a correspondingly high catching current during the armature movement. Thus, once the armature moves past the center position, it comes to rest against the momentarily catching electromagnet as a result of the magnetic force and counter to the force of the associated restoring spring.
The control of the electromagnetic actuator is dependent upon the operating data from the piston-type internal combustion engine that is available to the engine control, essentially involving the load requirement and the speed. For example, if the cylinder valve is in the closed position where the armature rests against the closing magnet, the armature is essentially controlled in dependence on the time, meaning via the engine control and by taking into consideration the crankshaft position and the parameters from the specified load, which respectively define the opening moment and the closing moment for the cylinder valve. The armature movement is started by switching off the relatively low holding current, so that during a predetermined interval after the holding current is switched off, the catching current on the catching electromagnet can be switched on. The time interval for this can be predetermined with the aid of previously obtained empirical data or even theoretical data.
If the catching current is switched on, the magnetic force increases progressively with increasing approach of the armature to the pole face of the catching electromagnet and a constant supply of current, while the opposite directed force of the restoring spring increases only linearly. As a result, the armature movement accelerates during the end phase just prior to the impact with the pole face of the catching electromagnet, thereby leading to a strong impact between armature and pole face. In many respects, this leads to disadvantages, for example by stimulating structure-born sounds and airborne sounds that result in noise development. In order to avoid this, an attempt is made to control the catching current by reducing it shortly before the armature impacts with the pole face of the respective catching electromagnet, wherein the armature approach is detected via a sensor arrangement. The reduction can occur in such a way that when the armature reaches a predetermined position near the pole face, a corresponding control signal is emitted or the armature movement is detected at this close-in range. By way of the engine control or a separate current regulation for the actuator, these approach values can then be used to reduce the catching current in such a way that the armature touches down softly on the pole face with a speed that is only slightly above xe2x80x9czero.xe2x80x9d The respective electromagnet therefore only needs to be admitted with the low holding current.
However, these known controls are inherently very rigid. On the one hand, they do not take into account the multitude of external interference forces that affect the system consisting of armature and cylinder valve. On the other hand, the noise development is at best minimized, but is not removed.
Thus, it is the object of the invention to create a method, which permits a much more exact control of an electromagnetic actuator and avoids the development of noise.
This object is solved according to the invention with a method for controlling an electromagnetic actuator for actuating a cylinder valve in a piston-type internal combustion engine. The actuator is provided with two spaced-apart electromagnets, between which an armature acting upon the cylinder valve can move back and forth between the pole faces of the two electromagnets, with a predetermined lift between an open position and a closed position of the cylinder valve and counter to the force of at least one restoring spring. A control is used to alternately admit the electromagnets with a catching current, and a sensor arrangement is used to detect the armature lift during its movement from one pole face to another. Thus, the current supply to the armature of the catching electromagnet is triggered via the control and in dependence on the detected actual armature lift values, such that the armature moves with a speed tending toward xe2x80x9czeroxe2x80x9d at a predetermined distance range to the pole face of the respective catching electromagnet. At the end of the lift, the catching electromagnet is supplied with only enough current for the armature to be held suspended at a short distance in front of the pole face.
In addition to the point in time for switching off the holding current, the expression xe2x80x9cactual armature lift valuesxe2x80x9d refers to at least the detection of the respective end position of the armature and, if necessary, the detection of its speed and its acceleration. In addition to detecting the position and depending on the type of sensor arrangement, the speed can be detected directly or can be derived from the derivation of the path based on the time resulting from the position detection, in the same way as for the acceleration.
The term xe2x80x9carmature liftxe2x80x9d within the meaning of the method according to the invention is defined by the path of the cylinder valve between its closed position and its opened position, that is to say without the armature leaving its support on the cylinder valve shaft because of a valve play. The distance between the two pole faces relative to each other is longer than the armature lift by approximately the measure of a valve play.
By dividing the armature movement into three phases, the physical characteristics of the actuator, meaning its individual mechanical characteristics as well as the characteristics changed by the operation of the piston-type internal combustion engine, are taken into account. During the first phase, the armature movement is only xe2x80x9cobserved,xe2x80x9d permitting the detection of the energetic starting position of the armature movement. This position is essentially predetermined through the actual point in time of separation from the pole face, as well as the force of the restoring spring that accelerates the armature on the one hand and the counteracting frictional forces and gas-pressure forces on the other hand. In the region close to the electromagnet, the energy losses in the mechanical system, caused by the remaining field that is effective in opposite direction, must of necessity be added when the armature separates. These negative electromagnetic force influences can still be minimized by using an armature with low eddy currents and/or by adding a current with a different polarization, which generates a magnetic field that has the effect of rejecting the armature.
However, as soon as the armature has separated noticeably from the pole face of the previously holding electromagnet, it becomes nearly impossible to influence the armature either through supplying a corresponding current to the previously holding electromagnet, or through supplying current at an earlier time to the catching electromagnet with a current intensity that is justifiable with respect to the energy expenditure. The armature reaches its maximum speed when passing through the center position. In that region, outer influences such as the cylinder inside pressure, frictional influences or even actuator parameters can affect the armature movement.
If, as specified in the method according to the invention, the actual armature lift values are detected via the sensor arrangement at least in the respective end position, the current supply for triggering the catching electromagnet can be controlled toward the end of the armature lift, such that during a predetermined distance range, a so-called xe2x80x9ctarget window,xe2x80x9d the armature moves with a speed tending toward xe2x80x9czeroxe2x80x9d as well as an acceleration moving toward xe2x80x9czero.xe2x80x9d At the end of the armature lift, the holding current supply is controlled such that the armature is held suspended, without the armature making contact with the pole face. In addition, the option of an individual adaptation of the current supply to the respective catching electromagnet is provided, taking into account the external interference influences that affect the armature during the movement. It is sufficient in that case if these predetermined values for speed and acceleration are reached at a specifiable, long distance range from the pole face.
The movement phase that starts when the target window is reached is characterized by a low armature speed and a high force effect of the catching magnet. Thus, a controlled guidance of the armature counter to the force of the restoring spring is possible during this phase until the end of the armature lift by controlling the current supply to the catching magnet, thereby ensuring that the armature is preferably held at a short distance to the pole face.
A soft touchdown of the valve on the valve seat is initially made possible through the controlled influencing of the armature movement in the end phase, during the closing movement and with a corresponding adjustment of the armature lift if a valve play exists. Following an at best slight separation of the armature from the valve, the armature itself is then held suspended in front of the pole face of the catching magnet, thereby ensuring that the valve is held with the full force of the closing spring in the closed position on its valve seat.
The current supplied to the electromagnets can respectively be regulated by controlling the voltage present at the catching magnet. Using a voltage control in place of a current control makes it possible to effect the necessary control actions faster and with more accuracy. The current drops relatively slowly even after the voltage is switched off and accordingly increases relatively slowly if a voltage is added. The voltage and current are advantageously supplied by making use of the on-board system for the piston-type internal combustion engine.